Method of Forming an Integrated Circuit

ABSTRACT

A method of forming an integrated circuit includes forming a patterned mask layer on a material layer, wherein the patterned mask layer has a plurality of first features, and a first distance between adjacent first features of the plurality of first features. The method further includes patterning the material layer to form the first features in the material layer. The method further includes increasing the first distance between adjacent first features of the plurality of first features to a second distance. The method further includes treating portions of the material layer exposed by the patterned mask layer. The method further includes removing the patterned mask layer; and removing non-treated portions of the material layer.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No.15/940,430, filed Mar. 29, 2018, which is a continuation of U.S.application Ser. No. 15/583,315, filed May 1, 2017, now U.S. Pat. No.9,934,971, which is a continuation of U.S. application Ser. No.14/711,842, filed May 14, 2015, now U.S. Pat. No. 9,640,398, which is acontinuation of U.S. application Ser. No. 14/304,022, filed Jun. 13,2014, now U.S. Pat. No. 9,059,085, which is a continuation of U.S.application Ser. No. 13/277,552, filed Oct. 20, 2011, now U.S. Pat. No.8,772,183, issued Jul. 8, 2014, the disclosures of which areincorporated by reference herein in their entireties.

TECHNICAL FIELD

The disclosure relates generally to integrated circuit fabricationmethods and, more particularly, to a method of fabricating an integratedcircuit with a reduced pitch.

BACKGROUND

Integrated circuits are commonly used to make a wide variety ofelectronic devices, such as memory chips. One aim in production is toreduce the size of integrated circuits, so as to increase the density ofthe individual components and consequently enhance the functionality ofan integrated circuit. The minimum pitch on an integrated circuit (theminimum distance between the same points of two adjacent structures ofthe same type, e.g., two adjacent gate conductors) is often used as arepresentative measure of the circuit's density. The feature width issometimes referred to herein as F, and the width of the space betweenfeatures is sometimes referred to herein as S.

Increases in circuit density often are limited by the resolution of theavailable photolithographic equipment. The minimum size of features andspaces that a given piece of photolithographic equipment can produce isrelated to its resolution capability. If one tries to define features ina photoresist which are smaller than the machine's minimum feature size,then the photoresist regions exposed to radiation may fail to correspondto the mask plate pattern, resulting in the photoresist features notbeing accurately reproduced.

Some attempts have been made to try to reduce the pitch of an integratedcircuit device below that of the minimum pitch producedlithographically, but these methods are difficult to control and showvarying results.

In view of the drawbacks of the prior methods, it is necessary toprovide a method that can reduce the pitch in a device below thatproducible by the lithographic process.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will be described with reference to theaccompanying figures. It should be understood that the drawings are forillustrative purposes and are therefore not drawn to scale.

FIG. 1 is a flow chart of a method of forming a structure of anintegrated circuit according to one or more embodiments of thisdisclosure.

FIGS. 2 to 7 are cross-sectional views showing various stages duringfabrication of a structure according to the method in FIG. 1.

DETAILED DESCRIPTION

The making and using of illustrative embodiments are discussed in detailbelow. It should be appreciated, however, that the disclosure providesmany applicable inventive concepts that can be embodied in a widevariety of specific contexts. The specific embodiments discussed aremerely illustrative and do not limit the scope of the disclosure.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “over” another element, it can bedirectly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “beneath” or “under” another element, it can bedirectly beneath or under the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly beneath” or “directly under” another element, there are nointervening elements present.

As used herein, a particular patterned layer is “used as a mask” for aparticular process step if it is the top layer present when theparticular process step is performed, or if it is only an intermediatelayer present when the particular process step is performed, as long asany superposing layers are patterned the same as or more narrowly thanthe particular layer. In other words, as used herein, if the structureincludes two patterned layers, then each of them individually, as wellas both of them together, are all considered herein to act as a “mask”for the particular process step. The presence of a superposing layerhaving the same or narrower pattern as the particular layer does notprevent the particular layer from being “used as a mask” for theparticular process step.

The term “substrate” as described herein, refers to a semiconductorsubstrate on which various layers and integrated circuit components areformed. The substrate may comprise silicon or a compound semiconductor,such as GaAs, InP, Si/Ge, or SiC. Examples of layers may includedielectric layers, doped layers, metal layers, polysilicon layers andvia plugs that may connect one layer to one or more layers. Examples ofintegrated circuit components may include transistors, resistors, and/orcapacitors. The substrate may be part of a wafer that includes aplurality of semiconductor dies fabricated on the surface of thesubstrate, wherein each die comprises one or more integrated circuits.The semiconductor dies are divided by scribe lines (not shown) betweenadjacent dies. The following process steps will be performed on each ofthe semiconductor dies on the surface of the substrate.

FIG. 1 is a flow chart of a method 100 of forming a structure of anintegrated circuit according to various embodiments of this disclosure.The method 100 begins at operation 101 where a first material layer isprovided. At operation 102, a second material layer is formed on thefirst material layer. At operation 103, a patterned mask layer is formedon the second material layer. The patterned mask layer has a pluralityof first features with a first pitch P₁. At operation 104, the secondmaterial layer is patterned by using the patterned mask layer as a mask.The first features are formed in the second material layer and a portionof a top surface of the first material layer is exposed. At operation105, the patterned mask layer is trimmed after patterning the secondmaterial layer. A trimmed patterned mask layer is formed. At operation106, a plurality of dopants is introduced into the second material layernot covered by the trimmed patterned mask layer. Various doped regionswith a second pitch P₂ are formed in the second material layer. Thesecond pitch P₂ is smaller than the first pitch P₁. At operation 107,the trimmed patterned mask layer is removed to expose various un-dopedregions in the second material layer. At operation 108, the un-dopedregions are selectively removed to form a plurality of second featuresin the second material layer. The plurality of second featurescorresponds to the respective doped regions in the second materiallayer. The plurality of second features has the second pitch P₂.

Referring to the drawings, FIGS. 2 to 7 depict cross-sectional viewsshowing various stages during fabrication of a structure according tothe method in FIG. 1.

Referring to FIG. 2, a first material layer 203, a second material layer205 and a patterned mask layer 207 are sequentially formed over thesubstrate 201. The layers 203, 205 and 207 are patterned, as will befurther discussed below, to form one or more features over the substrate201. It is understood that any desired feature may be patterned in thematerial layers, such as lines, gate structures and shallow trenchisolations (STIs), etc.

The first material layer 203 may include a dielectric layer (alsoreferred to as dielectric layer 203) or a metal layer (also referred toas metal layer 203) formed over the substrate 201 by any suitableprocess, such as chemical vapor deposition (CVD) and physical vapordeposition (CVD). The dielectric layer 203 may comprise silicon oxide,silicon oxynitride, silicon nitride, a high-k dielectric layercomprising hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO), hafniumsilicon oxynitride (HfSiON), hafnium tantalum oxide (HMO), hafniumtitanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), transitionmetal-oxides, transition metal-nitrides, transition metal-silicates,metal aluminates, zirconium silicate, zirconium aluminate, zirconiumoxide, titanium oxide, aluminum oxide, hafnium dioxide-alumina(HfO₂—Al₂O₃) alloy, and/or combinations thereof. The metal layer 203 maycomprise aluminum, copper, titanium, tantilum, tantalum nitride, nickelsilicide, cobalt silicide, TaC, TaSiN, metal alloys, and/or combinationsthereof.

The second material layer 205 is formed over the first material layer203 by any suitable process, such as chemical vapor deposition (CVD). Inone example, the second material layer 205 comprises a silicon layerincluding a polysilicon layer, a single crystalline silicon layer or anamorphous silicon layer. The second material layer 205 that may be usedas a mask layer for the underlying first material layer 203 for thefollowing etching process. In other words, the second material layer 205has a higher etch resistance than the first material layer 203 duringthe first material layer 203 etching process. The second material layer205 is formed to any suitable thickness. For example, the secondmaterial layer 205 has a thickness of in a range approximately 300 to2000 Å.

Next, the patterned mask layer 207 is formed over the second materiallayer 205. In one embodiment, the patterned mask layer 207 comprises aphoto resist layer (also referred to as photo resist layer 207). Theprocesses may include photoresist coating (e.g., spin-on coating), softbaking, mask aligning, exposure, post-exposure baking, developing thephotoresist, rinsing, drying (e.g., hard baking), and/or combinationsthereof. The patterned mask layer 207 has a plurality of first features209 with a first pitch P₁ formed over the second material layer 205. Thefirst pitch P₁ is the minimum distance between the same points of twoadjacent first features 209. The first pitch P₁ equals a width F1 of thefirst feature 209 plus a first space S1 between adjacent the firstfeatures 209.

In another embodiment, various imaging enhancement layers are formedunder photo resist layer 207 to enhance the pattern transfer of thefirst features 209 to the underlying layers. The imaging enhancementlayer may comprise a tri-layer including a bottom organic layer, amiddle inorganic layer and a top organic layer. The imaging enhancementlayer may also include an anti-reflective coating (ARC) material, apolymer layer, an oxide derived from TEOS (tetraethylorthosilicate),silicon oxide, or a Si-containing anti-reflective coating (ARC)material, such as a 42% Si-containing ARC layer.

In yet another embodiment, the patterned mask layer 207 comprises a hardmask layer. The hard mask layer comprises an oxide material, siliconnitride, silicon oxynitride, an amorphous carbon material, siliconcarbide or tetraethylorthosilicate (TEOS). The patterned hard mask layeris formed by defining the first features 209 in an overlying patternedphoto resist layer. The patterned photo resist layer is used as a maskfor etching the underlying hard mask layer. After etching, the firstfeatures 209 are formed in the patterned hard mask layer and thepatterned photo resist layer is removed.

Referring to FIG. 3, the second material layer 205 is patterned by usingthe patterned mask layer 207 as a mask. The first features 209 in thepatterned mask layer 207 are transferred to the second material layer205 by etching the second material layer 205. In this embodiment, apolysilicon layer is used as the second material layer 205. Thepolysilicon layer is etched with a plasma process in a Cl₂/HBr/O₂ambient environment. A portion of a top surface 211 of the firstmaterial layer 203 is exposed after the polysilicon layer etchingprocess. During the second material layer 205 etching process, the firstmaterial layer 203 has a higher etch resistance than the second materiallayer 205. Less of the first material layer 203 is consumed compared tothe second material layer 205 in this etching process. Most of the firstmaterial layer 203 remains.

Referring to FIG. 4, the patterned mask layer 207 is trimmed to form atrimmed patterned mask layer 208. In this embodiment, a patterned photoresist layer is used as the patterned mask layer 207. The photo resistlayer is etched with a plasma process in a HBr/O₂ ambient environment toform the trimmed patterned mask layer 208. The trimmed patterned masklayer 208 has a plurality of features 210 with a pitch P_(T) formed overthe first features 209 of the second material layer 205. The first spaceS₁ between adjacent the first features 209 in the patterned mask layer207 is widened to a space S_(T) between adjacent features 210 in thetrimmed patterned mask layer 208. A width F_(T) of the features 210 isless than the width F₁ of the first features 209.

Referring to FIG. 5, a plurality of dopants 213 is introduced into thesecond material layer 205 not covered by the trimmed patterned masklayer 208 to form doped regions 215 in the second material layer 205.Namely, the features 210 in the trimmed patterned mask layer 208 areused as a mask to form un-doped regions 217. The width F_(T) of thefeatures 210 substantially equals a space S₂ between adjacent dopedregions 215. A width F₂ of each doped region 215 substantially equalsthe difference between the width F₁ and the width F_(T) divided by two.The sum of the width F₂ and the space S₂ (or the width F_(T)) equals asecond pitch P₂ for doped regions 215. Since the width F_(T) is lessthan the width F₁, the second pitch P₂ is smaller than the first pitchP₁.

In one example, the second material layer 205 is a polysilicon layer.The plurality of dopants 213 is substantially vertically implanted intothe polysilicon layer. The dopants may include As, P, B, C, N, Si, Ge orBF₂. A dosage of the dopants is substantially higher than 1E15 ion/cm².The ion implantation creates different etching removal rates for theun-doped regions 217 and the doped region 215 in following removalprocess. The un-doped regions 217 may be selectively removed.Advantageously, since various dopants 213 are substantially verticallyimplanted, the second features 210 are accurately transferred from thetrimmed patterned mask layer 208 into un-doped regions 217 in the secondmaterial layer 205. Edges of the un-doped regions 217 (also edges ofdoped regions 215) are vertically aligned with the correspondingsidewalls of the second features 210 in the trimmed patterned mask layer208.

Referring to FIG. 6, the trimmed mask layer 208 is removed to exposeun-doped regions 217 in the second material layer 205. In one example,the trimmed mask layer 208 is a photo resist layer. The photo resistlayer may be ashed in an oxygen ambient environment.

Referring to FIG. 7, the un-doped regions 217 are selectively removed toform a plurality of second features in the second material layer 205.The second features are corresponding to the respective doped regions215. The un-doped region 217 has a higher etching removal rates than thedoped regions 215 has in the removal process. In one embodiment, thesecond material layer 205 is a silicon layer. The un-doped regions 217may be selectively removed in an etchant including tetramethyl ammoniumhydroxide (TMAH), tetrabutylphosphonium hydroxide, tetraphenylarsoniumhydroxide, KOH, NaOH or NH₄OH. When the etchant is TMAH, the etchantsolution is in a range of about 1 to about 10 weight percent of TMAH inde-ionized water to create a shaped image of the second features in thesecond material layer 205. In another embodiment, the un-doped regions217 may be removed by a dry etching process. The dry etching process hasa higher etching removal rates for un-doped region 217 than doped region215. After the un-doped region 217 removal process, the pattern of thedoped regions 215 is transferred to the second features in the secondmaterial layer 205. A width, a space and a pitch of the second featuressubstantially equal to the width F₂, the space S₂ and the pitch P₂ ofthe doped regions 215, respectively. The second features with a narrowpitch are fabricated.

In other embodiments, the first material layer 203 is etched by usingthe plurality of the second features in the second material layer 205 asa mask for fabricating the narrow pitch pattern in the first materiallayer 203.

Note that in all of the above embodiments, the feature narrowing processdescribed herein can be repeated if desired, assuming appropriatematerials are used in the starting structure of FIG. 2, and substrate201 include appropriate sub-layers superimposing the bulk supportmaterial. The repeated feature narrowing process can be thought of asbeing constructed by adding a second instance of the process stepsdescribed above either before or after the first instance describedabove.

In the above embodiments, the doped regions 215 are formed at edgeportions of the first features 209 in the second material layer 205 bymeans of processes which introduce a plurality of dopants into thesecond material layer 205. These processes can be implantation orthermal diffusion processes, as in the above-described embodiments, orcan be another form of chemical reaction or inter-diffusion reaction inother embodiments. Any process that creates different etching removalrates for un-doped region 217 and doped region 215 will suffice, so longas the impact of the process on other materials in the structure isinsignificant or otherwise accommodated.

In addition, it will be appreciated that the process of trimming thepatterned mask layer 207 has the effect of reducing the width of thefeatures in the second material layer 205. The following dopantintroducing process replaces the volume of the second material layer 205with a volume of doped regions 215 at edge portions of the firstfeatures 209. The final second feature has a width F₂ that is less thanthe starting width F₁ of the first feature 209.

In one embodiment, the first features 209 are formed in a regularrepeated pattern of the width F₁ and one-third of the width F₁ for thespace S₁, and the process can be used to form a new regular repeatedpattern of doped regions 215 (also the second features). The dopedregion 215 has equal width F₂ and space S₂. The width F₂ is alsosubstantially one-third of the width F₁. The space S₂ is alsosubstantially the space S₁. Hence, the second pitch P₂ is substantiallyone-half of the first pitch P₁. This can be accomplished by using atrimming process in which F_(T)=⅓ F₁=⅓ S_(T).

In another embodiment, the first features 209 are formed in a regularrepeated pattern of the width F₁ and one-half of the width F₁ for thespace S₁, and the process can be used to form a new regular repeatedpattern of doped regions 215 (also the second features). The width F₂substantially equals one-quarter of the width F₁. The space S₂substantially equals the space S₁. Hence, the second pitch P₂ issubstantially one-half of the first pitch P₁. This can be accomplishedby using a trimming process in which F_(T)=½ F₁=½ S_(T).

In other embodiments, F_(T) can be greater or less than ⅓ F₁, and/orF_(T) can be greater or less than ⅓ S_(T), and/or the original patternedmask layer 207 may not be formed in regular pattern of equal width andspace. Variations such as these and others can be used to producevarious different sub-lithographic features patterns as desired in theresulting integrated structure.

Various embodiments of the present invention may be used to improve themethod of fabricating an integrated circuit with a reduced pitch. Forexample, during the processes for pitch reduction, only one lithographprocess is needed to define the starting features in the patterned masklayer. There is no overlay issue that comes from the features formed byanother lithograph process. In other pitch reduction methods, thepatterned mask layer may be composed of two similar initial features.Each feature has it own film stack. When the patterned mask layer isused as a mask to perform an etching process on a specific layer, theresulting features in the specific layer will generate two groups due tothe influence from the film stack of the initial features. The resultingfeatures have different dimensions in the completed products. The deviceperformance and yield are thus hard to control. The processes in thisdisclosure are performed in a way of pattern transference in the samestacking film. The resulting features have the identical dimension. Thedevice performance and yield of the completed products are easilycontrolled. The disclosed embodiments increase the flexibility toallocate different products for the production line.

One aspect of this description relates to a method of forming anintegrated circuit. The method includes forming a patterned mask layeron a material layer, wherein the patterned mask layer has a plurality offirst features, and a first distance between adjacent first features ofthe plurality of first features. The method further includes patterningthe material layer to form the first features in the material layer. Themethod further includes increasing the first distance between adjacentfirst features of the plurality of first features to a second distance.The method further includes treating portions of the material layerexposed by the patterned mask layer. The method further includesremoving the patterned mask layer; and removing non-treated portions ofthe material layer.

Another aspect of this description relates to a method of forming anintegrated circuit. The method includes forming a patterned mask layeron a first material layer, the patterned mask layer having a pluralityof first features with a first pitch, wherein the material layer is overa second material layer. The method further includes transferring theplurality of first features to the first material layer; and increasingthe first pitch in the patterned mask layer to a second pitch. Themethod further includes treating portions of the first material layerexposed by the patterned mask layer. The method further includesremoving the patterned mask layer; and removing non-treated portions ofthe first material layer to form a plurality of second features having athird pitch.

Still another aspect of this description relates to a method of formingan integrated circuit. The method includes forming a patterned masklayer on a silicon-containing layer, the patterned mask layer having aplurality of first features with a first pitch. The method furtherincludes patterning the silicon-containing layer using the patternedmask layer; and trimming the patterned mask layer to form a trimmedpatterned mask layer. The method further includes implanting dopantsinto the silicon-containing layer exposed by the trimmed patterned masklayer. The method further includes removing the trimmed patterned masklayer; and removing un-doped portions of the silicon-containing layer toform a plurality of second features having a second pitch.

Although the embodiments and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein.

What is claimed is:
 1. A method comprising: forming a conductive layerover a substrate; forming a silicon-containing layer on the conductivelayer; forming a patterned hard mask layer on the silicon-containinglayer; patterning the silicon-containing layer using the patterned hardmask layer as a mask; treating portions of the patternedsilicon-containing layer exposed by the patterned hard mask layer;removing the patterned hard mask layer; and removing non-treatedportions of the patterned silicon-containing layer, wherein the removingof the non-treated portions of the patterned silicon-containing layerexposes a portion of the conductive layer.
 2. The method of claim 1,wherein the silicon-containing layer includes polysilicon material andthe conductive layer includes a metal material.
 3. The method of claim2, wherein the metal material is selected from the group consisting ofaluminum, copper, titanium, tantalum, tantalum nitride, nickel silicide,cobalt silicide, TaC and TaSiN.
 4. The method of claim 1, furthercomprising trimming the patterning hard mask layer to expose theportions of the patterned silicon-containing layer prior to treatingportions of the patterned silicon-containing layer exposed by thepatterned hard mask layer, and wherein the removing of the patternedhard mask layer includes removing the trimmed patterned hard mask layer.5. The method of claim 1, wherein the removing of the patterned hardmask layer exposes the non-treated portions of the patternedsilicon-containing layer.
 6. The method of claim 1, wherein the treatingof the portions of the silicon-containing layer exposed by the patternedhard mask layer includes doping the portions of the silicon-containinglayer.
 7. The method of claim 6, wherein the doping of the portions ofthe silicon-containing layer including incorporating a dopant into theportions of the silicon-containing layer, the dopant selected from thegroup consisting of As, P, B, C, N, Si, Ge and BF₂.
 8. A methodcomprising: forming a first material layer on a substrate, the firstmaterial layer having a first material composition; forming a patternedphotoresist layer on the first material layer; patterning the firstmaterial layer by using the patterned photoresist layer as a mask whilea topmost surface of a portion of the patterned photoresist layer isexposed; changing the material composition of a first portion of thepatterned first material layer to have a second material compositionwhile a second portion of the patterned first material layer is coveredby the patterned photoresist layer, wherein after the changing of thematerial composition of the first portion of the patterned firstmaterial layer to have the second material composition, the secondportion of the patterned first material layer still has the firstmaterial composition; removing the patterned photoresist layer; andremoving the second portion of the patterned first material layer. 9.The method of claim 8, further comprising forming a second materiallayer on the substrate prior to forming a first material layer on asubstrate, the second material layer being formed of a differentmaterial than the first material layer.
 10. The method of claim 9,wherein the second material layer is formed of a material selected fromthe group consisting of a dielectric material and a metal material. 11.The method of claim 9, wherein a portion of the second material layer isexposed after the removing of the second portion of the patterned firstmaterial layer.
 12. The method of claim 8, further comprising trimmingthe patterned photoresist layer prior to the changing of the materialcomposition of the first portion of the patterned first material layer.13. The method of claim 12, wherein the first portion of the patternedfirst material layer is exposed after the trimming of the patternedphotoresist layer.
 14. The method of claim 8, wherein the second portionof the patterned first material layer is exposed after the removing ofthe patterned photoresist layer.
 15. The method of claim 8, wherein theremoving of the second portion of the patterned first material layeroccurs after the removing of the patterned photoresist layer, andwherein the removing of the second portion of the patterned firstmaterial layer includes performing a dry etching process.
 16. A methodcomprising: forming a first material layer on a substrate; forming asecond material layer on a substrate; forming a first feature on thesecond material layer; patterning the second material layer by using thefirst feature as a mask, wherein the patterning of the second materiallayer by using the first feature as the mask includes removing some ofthe second material layer to thereby expose at least a portion of thefirst material layer; patterning the first feature to form a patternedfirst feature; changing a first portion of the patterned second materiallayer that is exposed by the patterned first feature from a firstmaterial composition to a second material composition while a secondportion of the patterned second material layer is covered by thepatterned first feature; removing the patterned first feature; andremoving the second portion of the patterned second material layer,wherein the first portion of the second material layer remains after theremoving of the second portion of the patterned second material layer.17. The method of claim 16, wherein the forming of the first materiallayer on the substrate includes forming the first material layerdirectly on the substrate, wherein the forming of the second materiallayer on the substrate includes forming the second material layerdirectly on the first material layer, and wherein the forming of thefirst feature on the second material layer includes forming the firstfeature directly on the second material layer.
 18. The method of claim16, wherein the removing of the second portion of the second materiallayer includes performing an etching process with an etchant selectedfrom the group consisting of tetramethyl ammonium hydroxide (TMAH),tetrabutylphosphonium hydroxide, tetraphenylarsonium hydroxide, KOH,NaOH and NH₄OH.
 19. The method of claim 16, wherein a portion of thefirst material layer is exposed after the removing of the second portionof the patterned second material layer.
 20. The method of claim 16,wherein first material layer includes a conductive material, and whereinthe second material layer includes silicon.